As is well known, large numbers of integrated circuit elements are often fabricated on a single semiconductor substrate wafer. In order to maximize the yield of the wafer, i.e., the percentage of non-defective circuit elements, it is essential to precisely control the processing environment. This is of particular importance during photolithographic processing. As an initial step in such processing the wafer is coated with a photosensitive layer using a liquid chemical such as photoresist. A desired circuit pattern is then defined by:
(i) selectively exposing the photosensitive layer by illuminating a mask using light of a specific wavelength and intensity, and PA1 (ii) removing unexposed portions of the photosensitive layer using, for example, chemical etchants.
Since the portion of an integrated circuit underlying a coated area of the wafer can be destroyed if exposed to chemical etchants, acceptable wafer yields require that the coated area be virtually free of discontinuities. Such discontinuities are known to arise, for example, as a consequence of air bubbles present in photoresist dispensing systems. Hence, wafers tend to be destroyed, and wafer yields thereby degraded, in proportion to the relative amount of air present within the dispensed photoresist.
In manual dispensing systems the photoresist liquid is generally drawn from a small bottle or container which must be replaced when empty. However, during manual replacement of the photoresist container there is a tendency for air to become trapped in various supply lines of the dispensing system. That is, if a container becomes completely empty before a change is made air will be introduced into the dispensing system. This can cause discontinuities in the photoresist layer, and requires that a purging procedure be performed to rid the system of the unwanted air. Such a purging operation wastes residual photoresist present within the dispensing lines and reduces production efficiency. In addition, unwanted air within the dispensing system can result in particle formation due to oxidation of the surface of the photoresist. As is well known, the presence of such particles within the photoresist can create defects or discontinuities in the circuit lines defined on the wafer.
As photoresist and other processing liquids have become more expensive, it has become apparent that the waste of processing liquids during manual dispensing operations can significantly increase the cost of wafer processing.